Time to reassess my objective. What I’m really interested in is the cheapest way to place my order. That means what I should be focusing on at this point is the price discrepancy among the different combos, rather than the absolute total amount.

With this in mind, I scan the price list again. The presence of some round dollar amounts and a healthy ‘number sense’ lead me to discover something. The difference in cost between the Fries + Coke combo and Wedges + Coke combo for all mains is $0.80.

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That tells me there is no cost difference between the order configurations below:

  1. Main A a la carte + Main B-Fries-Coke combo + Main C-Wedges-Coke combo
  2. Main A a la carte + Main C-Fries-Coke combo + Main B-Wedges-Coke combo

Thus, the problem is now reduced to identifying the main that I should order a la carte.

I shift my attention to the next type of price difference: main a la carte and Fries-Coke combo (or Wedges-Coke combo).

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It’s now clear which main that I should order a la carte. If like me, you pick correctly, you should be paying only $14.40. If you pick incorrectly, you could end up paying $1.45 more!

Parents, the next time you visit McDonald’s, you might want to delegate the ordering to your children to give them a mental maths workout.

Note to self: To discuss with Mr Sim about adapting this scenario in an upcoming Maths lesson…

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